tee/optee/shm_pool: fix application of sizeof to pointer

sizeof when applied to a pointer typed expression gives the size of
the pointer.

./drivers/tee/optee/shm_pool.c:38:28-34: ERROR application of sizeof to pointer

This issue was detected with the help of Coccinelle.

Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: jing yangyang <jing.yangyang@zte.com.cn>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>